1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, which includes a step of heating a semiconductor substrate in a vacuum. The present invention also relates to a tool for manufacturing a semiconductor device, which is used in the above step, and a relevant apparatus for manufacturing a semiconductor device.
Priority is claimed on Japanese Patent Application No. 2006-236606, filed Aug. 31, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
In comparison with silicon (Si), wide band gap (WBG) semiconductors such as silicon carbide (SiC) or gallium nitride (GaN) have a wider band gap, and thus are materials having superior voltage-resistant and heat-resistant characteristics. Therefore, they are anticipated as potential materials for power devices. The structures of WBG power devices, which are being actively researched and developed at present, are mainly classified into a Schottky barrier diode (SBD), a PiN junction diode (PiND), a MOS (metal oxide semiconductor) transistor, and a junction transistor. Among them, the structures of the SBD and the junction transistor will be explained.
FIG. 17 is a schematic plan view showing a planar structure of an SBD. FIG. 18 is a schematic sectional view showing a sectional structure of the SBD along line A-A in FIG. 17. A drift layer 110, which is an n-type high-resistance layer, is formed on one face of a cathode layer 100, which is an n-type low-resistance layer. A Schottky electrode 120 is formed on the surface of the drift layer 110. A junction electrode 130 is formed on the surface of the Schottky electrode 120. In a part of the surface of the drift layer 110, a guard ring area 140, which is a p-type low-resistance layer, is formed so as to surround the Schottky electrode 120. An insulating layer 150 is formed in a manner such that it hangs over the guard ring area 140, the Schottky electrode 120, and the junction electrode 130. An Ohmic electrode 160 and a junction electrode 170 are formed on the other face of the cathode layer 100.
FIG. 19 is a schematic plan view showing a planar structure of a junction transistor. FIG. 20 is a schematic sectional view showing a sectional structure of the junction transistor along line B-B in FIG. 19. A drift layer 210, which is an n-type high-resistance layer, is formed on one face of a drain layer 200, which is an n-type low-resistance layer. A source area 220, which is an n-type low-resistance area, is formed in a part of the surface of the drift layer 210. A gate area 230, which is a p-type low-resistance area and surrounds the source area 220, is formed in a part of the surface of the drift layer 210. A source electrode 240 is formed on the source area 220, and a gate electrode 250 is formed on the gate area 230. A drain electrode 260 is formed on the other face of the drain layer 200.
As shown in FIG. 19, a number of the source electrodes 240 are arranged, which are isolated from each other. Each of the source electrodes 240 has an elongated form. Around each source electrode 240, the gate electrode 250 is provided as a control electrode. The on/off state of electric current flowing between the source electrode 240 and the gate electrode 260 is controlled using a signal applied to the gate electrode 250.
Generally, due to a difference in material for the substrate, known processes for manufacturing silicon devices cannot be directly applied to the processes for manufacturing the above-described WBG devices. Therefore, development of a novel processing technique is of great importance. A technique for forming a selective conductive area, such as the guard ring area 140 in FIG. 18, or the gate area 230 in FIG. 20, is an example of such a technique. WBG materials have a relatively small diffusion coefficient with respect to impurities, which are used for controlling the conductivity of the semiconductor. Therefore, when forming the guard ring area or the gate area as a selective conductive area, a thermal diffusion method used in the processes for silicon devices cannot be used. Accordingly, an ion implantation method is mainly used for controlling the conductive type.
In the first step of the ion implantation method, an element suitable for the relevant conductive type is ionized in a vacuum, and generated ions are accelerated through an electric field so as to have a desired velocity. The accelerated ions are injected into a WBG material. For example, with respect to silicon carbide, nitrogen (N) or phosphorus (P) may be an element suitable for the n-type, and aluminium (Al) or boron (B) may be an element suitable for the p-type. After only the ions are injected, (i) WBG crystals have defects due to the ion injection, and (ii) sufficient engagement between the injected element and the semiconductor crystals has not yet been established. Therefore, thermal processing at a high temperature is performed so as to repair and remove the defects in crystals, and to incorporate the injected element into the crystals. Such thermal processing is called “activation (or activation annealing)”.
During the activation, for example, a WBG substrate is disposed on a susceptor made of graphite, which is set in a quartz chamber, the periphery of the susceptor is covered with a heat insulating material, and the WBG substrate is heated by induction heating (from the outside of the quartz chamber) while using the graphite as a thermal source, so that the substrate reaches a desired temperature. Generally, the heating is performed in a rare gas such as argon, as an atmospheric gas. However, impurities included in the rare gas contaminate or corrode the surface of the WBG substrate, so that the surface of the WBG substrate becomes rough. Such roughness of the surface degrades the performance or characteristics of the semiconductor device. Therefore, it is desirable to reduce the surface roughness as much as possible.
Reduction of the surface roughness can be anticipated by using a vacuum atmosphere for the activation, instead of the rare gas. Generally, the highest purity with respect to presently-available gases is 99.9999%, and impurities of approximately 1 ppm are included therein. The partial pressure of impurities in a rare gas at the atmospheric pressure is 1×10−1 Pa. However, when using a vacuum apparatus in the current technique, 1×10−4 Pa or less (for impurities) can be reliably obtained with respect to the total gas pressure including rare gas pressures. Generally, a combination of a turbo molecular pump or a diffusion pump, and an oil-sealed rotary pump or a dry pump is generally used as such a vacuum apparatus. Accordingly, it is possible to make the amount of impurities, which cause damage on the surface of the WBG substrate, be 1/1000 or less in comparison with the case of not using a vacuum apparatus.
However, heating in a vacuum has a problem. Generally, there are three mechanism for heat transmission, such as radiation, convection, and transmission. In the radiation mechanism, light radiated from a higher-temperature object should be absorbed by a lower-temperature object. Here, WBG materials, having a wide band gap, cannot absorb such light generated by radiation, and thus are not heated. That is, radiated light passes through the WBG substrate without providing heat to the substrate, and thus it is very difficult to heat the WBG substrate.
FIG. 21 is a graph showing wavelength dependency of the light absorption rate of 4H—SiC (as an example of WBG materials), and also wavelength dependency of light intensity radiated from a heating source. Silicon carbide having a band gap of 3.2 eV can absorb light having a wavelength of 380 nm or shorter, however, it transmits light having a longer wavelength. On the other hand, light radiated from any heating source has a wavelength of 400 nm or higher even at 2000° C., so that it cannot heat silicon carbide.
When heating is performed in a rare-gas atmosphere, the convection and transmission mechanisms can be used. In the convention mechanism, the relevant susceptor heats a rare gas, which exists in a gap between the susceptor and the WBG substrate, and the heated gas heats the WBG substrate. In the transmission mechanism, heat is directly transmitted from the graphite susceptor to the WBG substrate. The amount of heat, which is directly transmitted from the graphite susceptor to the WBG substrate, considerably depends on the contact area between the susceptor and the WBG substrate. Although a face of the graphite susceptor contacts a face of the WBG substrate, a slight warp or unevenness is present in either face, actually. Therefore, both faces do not sufficiently contact each other only by disposing the WBG substrate on the susceptor. Accordingly, the convention mechanism mainly controls the heating in a rare-gas atmosphere. FIG. 22 is a schematic diagram showing a state in which a WBG substrate 1 is simply disposed on a susceptor 2e in conventional techniques.
On the other hand, when considering heating in a vacuum, heating cannot be executed using radiation as described above, or also by convection because there is no gas as a medium for transmitting heat. Therefore, the WBG substrate is heated only by the transmission mechanism. However, the contact area between the WBG substrate and the susceptor, either of which has a slight warp, is not constant due to an individual difference or a disposing manner of the substrate. Therefore, reproducible heat-transmitting performance cannot be obtained, and thus desired device performance or characteristics cannot be obtained. In order to solve this problem, Patent Document 1 discloses a method of depositing carbon or metal on the back face of a semiconductor substrate.    Patent Document 1: Japanese Utility Model Application, First Publication No. H5-29140.
As described above, conventional techniques have a problem such that it is difficult to heat a semiconductor substrate in a vacuum. In addition, the method disclosed in Patent Document 1 has a problem such that it is difficult to remove a deposited film after thermal processing, and also a problem such that the semiconductor substrate or a manufacturing line is contaminated by metal.